Methylmercury (MeHg) exposure from fish consumption during childhood may adversely affect developmental outcomes. The brain is not fully developed until halfway through the third decade of life and is therefore potentially susceptible to MeHg exposure well beyond the fetal period. It is currently not known if it is safe for children to eat fish. While epidemiological evidence supports the hypothesis that postnatal MeHg exposure is adversely associated with children's development, studies have not been specifically designed to study postnatal MeHg exposure. Nearly all studies have been cross-sectional evaluations in which MeHg exposure was measured only at the time of cognitive testing (convenience samples). No studies have employed the more definitive and powerful approach of longitudinal assessment of postnatal MeHg exposure in relation to cognitive outcomes throughout childhood and adolescence. We are in a position to study this issue longitudinally in the Republic of Seychelles where dietary MeHg exposure is among the highest in the world. In 1989-90 we enrolled a prospective cohort of 779 mother-infant pairs (?Main Cohort?). We have 24 years of postnatal exposure and neurodevelopmental data in this cohort, which has been characterized more extensively than any other cohort. In cross-sectional analyses we have found adverse associations between recent postnatal MeHg exposure and various developmental endpoints at different ages, but especially with measures of psychomotor and executive function, attention, and general intelligence at 9 years of age and older. These cognitive domains have their greatest development postnatally and are therefore more likely to be affected by postnatal MeHg exposure. These intriguing results need to be studied using longitudinal analyses that better address the variability in postnatal exposure and its impact on neurodevelopment over time. The aim of this proposed study is to examine the longitudinal associations of cumulative postnatal MeHg exposure with neurodevelopment. We hypothesize that greater accumulated postnatal exposure is adversely associated with neurodevelopmental outcomes in later childhood and adolescence. We will use a previously-developed cumulative exposure metric based on repeated measurement of children's hair MeHg values. So far, this metric has only been examined in relation to one developmental outcome at 9 years of age. We now have an additional 15 years of exposure and developmental data on this cohort. We will leverage these data and examine three cumulative postnatal exposure metrics (early life, late adolescent, and lifetime) in relation to cognitive domains that have shown adverse associations in cross-sectional analyses. Longitudinal models will evaluate the consistency of associations over time. Billions of people depend daily on the nutritional properties of fish. It is therefore critical to examine whether children's exposure to MeHg from consumption of fish is associated with adverse developmental consequences. This study will provide the most comprehensive assessment to date of the risks or safety of fish consumption in childhood.